The present invention relates to instruments and methods for probing wafers of digital devices, and more particularly to probing wafers of digital light-emitting devices.
Digital light-emitting devices are well known and widely used in many applications. Two such devices are vertical cavity surface emitting lasers (VCSELs) and light-emitting diodes (LEDs). These devices are fabricated on wafers with counts of over 20,000 individual devices on a three-inch diameter wafer being common. After fabrication on the wafer, the devices are cut into individual devices (called die) and packaged for use.
Manufacturers of light-emitting devices desire to make optical measurements of the individual devices before the wafers are cut into individual dies. The motivation for such measurements is that the characteristics of the devices can vary significantly across a single wafer. By mapping a wafer, a manufacturer can bin the die after cutting, separating the good from the bad and segregating the good ones by power output, wavelength, or other parameters. Performing this testing while the devices are still in wafer form permits the manufacturer to eliminate the bad ones before spending money to package them. Also, some manufacturers sell bare die to packagers, who want specifications on what they are buying.
Semiconductor manufacturers have been probing devices and circuits on wafers for many years, for example, measuring the electrical properties and performance of the integrated circuits on wafers. Instruments for probing digital devices are known as xe2x80x9cprobersxe2x80x9d and are manufactured and sold, for example, by Karl Suss America, Inc. of Waterbury Center, Vt. (www.suss.com).
Usually, information on two optical parameters is desiredxe2x80x94the total power and the spectral distribution. Currently, the spectral distribution of the light is measured using a fiber optic spectrometer mounted on the prober. The device can be used in conjunction with the microscope which is mounted on the prober for initial alignment of the wafer on the prober bench. The prober moves the wafer to sequentially align each light-emitting device with the fiber optic pick-up; the light-emitting device is activated; and the light is captured by the fiber optic pick-up. Unfortunately, problems arise in conjunction with the use of fiber optic pick-ups. First, the device to be tested must be precisely aligned with the pick-up. If not, a portion of the light output is lost (i.e. does not enter the pick-up), negatively impacting the accuracy of the measurement. Second, the fiber optic pick-up receives only a portion of the light even under optimal circumstances, because the light emitted from light-emitting devices diverges. This further detracts from the accuracy of the measurement.
The aforementioned problems are overcome in the present invention in which a light-integrating sphere is included in a wafer prober for more completely capturing the light outputted by each device. The sphere has a relatively large opening in conjunction with the previously used fiber optic pick-ups. Accordingly, a significantly higher percentage of the light from the device is captured by the sphere.
More specifically, the invention comprises a wafer prober on which an integrating sphere is mounted. In operation, the prober aligns at least one light-emitting device with the sphere, activates each device aligned with the sphere, and measures the light output of the activated device.
These and other objects, advantages, and features of the invention will be more readily understood and appreciated by reference to the detailed description of the preferred embodiment and the drawings.